The pinnacle came on Dec. 30, 1999, when Microsoft hit an intraday high share price of $119.94 (the equivalent of $45.57 today, adjusting for splits). That gave Microsoft a market capitalization of $618.9 billion, the highest ever recorded by a publicly traded company.

Microsoft now trades at a valuation of around $260 billion.

Apple (AAPL, Fortune 500) shares reached a record high of $644 in Tuesday morning trading, nudging the company just past the $600 billion line. Shares later slipped to $641.47, a nearly 1% rise from Monday's closing price

Despite its size, Apple is still one of the fastest growing technology companies. The company reported in January that its sales grew 73% last year. It also posted the second most profitable quarter in history for a U.S. company.

The stock has been soaring for the past three years, but went on a tear in 2012. Apple shares reached $500 for the first time in February, then topped $600 in March.

Apple had $127.8 billion in sales during the 2011 calendar year, putting it neck-and-neck with Hewlett-Packard (HPQ, Fortune 500), the nation's largest tech company by revenue. This year, Apple is on pace to become the biggest technology company in the world, measured by revenue, outpacing current global No. 1 Samsung.

When Cook took the helm in August from then-CEO Steve Jobs, Apple's board gave him an eye-popping grant of 1 million company shares. It came with a giant restriction: a 10-year vesting schedule. Cook is eligible to collect the first half of his stock package in 2016 and the rest in 2021, if he remains employed with Apple.

At the time of the grant, Cook's 1 million shares were worth $384 million. Apple's closing share price on Monday values them at $636 million.